1. Field of the Invention
This invention relates to apparatus for ascertaining a predetermined liquid level and in particular to such apparatus especially adapted for use in hostile environments, i.e., extremes of temperature or pressure.
2. Prior Art
In certain chemical laboratory or analytical apparatus, gases or vapors are passed through vessels in which very high temperatures or very high pressures exist or in which corrosive or other hostile conditions are found. Within those vessels, as a result of condensation or chemical reaction, for example, liquid drops from the gases or vapors form a body of liquid. From time to time it is desired to remove the accummulated liquid through an outlet valve in the vessel when a predetermined volume of it has been collected. In order to accomplish this automatically, it is highly useful to have associated with it, either within or without, apparatus for automatically detecting when the liquid attains a predetermined level. It is desirable to remove the accumulated liquid without disturbing the flow of the gases through the vessel.
In a typical example, there may be a flow of gases through a microreactor or micropilot system such as the Model 800 or Model 8000 marketed by Chemical Data Systems of Oxford, Penna. Through the micropilot or reactor vessel there may be a flow of gases into contact with a catalyst for generating a predetermined product or products. Sometimes it is not known how much of the product will be in the liquid phase relative to the amount in the gas phase. Sometimes gases or vapors will condense on the inner walls of the vessel. The balance of the system following the reactor may not be able to handle the liquid phase component. If the vessel is equipped with an automatically-operated outlet valve and some means for generating a signal when the liquid body rises to a predetermined level, that signal may be used to actuate the valve to remove a certain amount of the liquid in the body. The gas phase components may continue to pass through the vessel for further analysis downstream.
In the past, there have been a number of approaches to detection of liquid level. One of them involves the detection of a change in capacitance as the liquid rises to the predetermined level. This type of approach is not useful with certain types of liquids such as paraffines whose dielectric constant is extremely high. Nor is it practical when the gases in the vessel are subject to extreme or widely-varying pressures. The presence of bodies of viscous liquids may also impair the efficiency or even the utility of capacitance-type liquid level detectors.
Another known approach is a light or optical detection system. If this type of detection system is located within the vessel, its efficacy can be seriously impaired by the production of tarry or similar types of light-obstructing substances within the vessel. If the optical detection system is located externally, such types of material may condense or otherwise accumulate on the inner surface of the vessel and similarly obstruct accurate optical detection of the liquid level.
Still another approach is to use radioactive materials which emit sensing rays. The trouble with them is that they are very expensive and require approval by appropriate government authorities.
Another detection system involves the use of sonic or ultrasonic detectors. A sonic emitter may be placed within the vessel so that its waves are reflected back to a receiver from the top surface of the liquid body. However, different gases in such a vessel differently affect the velocity of the propagation of the sound waves leading to inaccurate readings. Variations in temperature and pressure may likewise introduce variables in the sensing system thereby making it difficult to use this approach where the ranges of such variations and the concentration of the various gases may not be known in advance.
The use of a float-ball assembly as the detection system within a highly-pressurized vessel also is not practical, because any device that would float on the surface of liquids having widely-varying densities must be very light and delicate and therefore could not withstand those high pressures.
Another alternative is to use a thermal detector which involves the placing of a heat generator within the liquid and a plurality of heat sensors located above it. However, since many vessels are associated with programmed heating cycles, this system cannot easily accommodate them. If the temperature is cycled, it is impossible to get the differential between the temperature of the liquid and the temperature of the gas necessary for the sensors to be operative. Sometimes the gas can be hotter than the liquid or vice-versa making detection accuracy impossible.
Other systems employ two metallic probes connected to external electrical circuits and depend upon the conductivity of the liquid to complete a circuit. They unfortunately are of value only with liquids that are electrolytes but there are many liquids which are not.
An approach that has proved useful for detecting the level of non-liquids such as granular, powdered or particulate material, e.g., grain or pelletized plastic material is the Endress and Hauser piezoelectric level switch Model LSM 1700. It employs one or more vibrating elements whose vibrations are damped as they come into contact with the pellets so that the decrease in the amplitude of the vibrations can be sensed. If this type of detector is employed with certain liquids, the vibrating elements may acquire a coating of the liquid and cause the resonant frequency of the elements to change. If the amount of the change in resonant frequency is not known in advance, compensation in the associated circuitry is not practical. This vulnerability to frequency variations renders such types of detection systems of little value for detecting liquid levels.
It is therefore among the objects of the present invention to provide:
(a) A system and method for detecting a predetermined level of liquid, especially in a hostile environment.
(b) A system and method for detecting a predetermined level of a liquid which is not an electrolyte.
(c) A liquid level detecting system and method capable of being used with enclosures subjected to programmed heating or pressure cycles.
(d) A liquid level detecting system for regulating the release of a liquid from an enclosure in which there are extremes of heat or pressure.
(e) A liquid level detecting system involving a probe having a vibrating element whose natural resonance frequency may be affected by hostile environments.